The invention relates to a variable speed drive to be connected between a rotating driving shaft and a rotating driven shaft and allowing a continuous variation of the transmission ratio between these two shafts.
Different implementations of variable speed drives have already been described, comprising intermediate alternating movements of variable amplitude which determine the transmission ratio.
FR-A-1.133.080 describes a variable drive comprising connecting rod/crank systems having torsion bar and freewheel. FR-A-2.125.083 describes a variable speed drive comprising a series of peripheral assemblies which oscillate by action of a cam and are coupled by freewheels to a driven shaft. The transmission ratio depends on the amplitude of the cyclic movement of the peripheral assemblies which themselves depend upon the eccentricity of the cam. Also, FR-A-2.200.933 describes a variable drive having variable ratio straight gears functioning as a square wave generator, the variation of the amplitudes of the square waves causing the variation in the transmission ratio.
Moreover, drives are known which comprise an angled bearing allowing the generation of intermediate alternating movements transmitted onto a driven shaft by a freewheels mechanism (see, for example, FR-A-2.538.532).
Nevertheless, and in spite of the age of these documents, the different devices that they describe have not been usable, in practice, because of their low performance. Indeed, in addition to the large losses caused by the many moving elements, their design theory has the consequence that the engendered alternating movements do not have the shape of continuous successive square signals. It is, therefore, necessary to associate a hydraulic coupling and/or a flywheel to the variable drive, which causes the assembly to be oversized. Moreover, the movements of the variable drive generate strong vibrations. Moreover, with these known variable drives, the transmission of an engine brake is not possible.
Also, the movements carried out at high frequency engender fatigue and vibration problems which are not taken into account in known variable drives. Also, the mechanical play of the moving elements accumulate, further reducing performances, functioning precision, and the behavior over time of these variable drives, notably for the variable drive described in FR-A-2.200.330.
Equally, the control of the functioning and in load of the variation in amplitude inducing the variation of the transmission ratio and, more generally, the control of the various functioning modes of the conventional variable drives, present problems which are not fully resolved. Indeed, these controls must be carried out with hydraulic control units which are expensive, complex, bulky and which consume a large amount of energy.